There is a good answer to why Dart was chosen as a language for Flutter: https://flutter.dev/docs/resources/faq#why-did-flutter-choose-to-use-dart

The main point of my article is not “they should use Kotlin instead”, but rather what features I lack in Dart that are present in Kotlin (and some other mainstream languages). If Dart team would be able to resolve them (in the same way as Kotlin does, or in some other) I would be more than happy to continue using Dart.